1. Field of the Invention
The present invention relates in general to temperature sensors, and in particular, to a new and useful optical pyrometer for measuring furnace gas temperatures.
2. Description of the Related Art
Prior devices have measured the temperature of entrained fly ash to determine gas temperature, limiting their application to only fuels with a high ash content. The present invention measures the temperature of actual constituent gases, thereby providing accurate temperature measurement with any fuel.
Radiation pyrometry, more commonly called optical pyrometry, measures the temperature of a substance by measuring the thermal radiation emitted by the substance.
Thermal radiation is a universal property of matter that is present at any temperature above absolute zero. Optical pyrometry utilizes the fact that the thermal radiation emitted by most substances is continuous over a spectral range of approximately 0.3 micron to 20 microns. This spectral range encompasses ultraviolet (UV) radiation, up to 0.38 micron; the visible (VIS) range, 0.38 to 0.78 micron; and infrared radiation (IR), 0.78 to 20.0 micron. IR radiation is further divided into three segments, near IR (0.78 to 3.0 micron), middle IR (3.0 to 6.0 microns), and far IR (above 6 microns).
The distribution of the thermal radiation of a substance over the spectral range is a function of both the temperature and emissivity of the substance. Higher temperatures shift the distribution toward the shorter wavelengths; lower temperatures shift the distribution toward the longer wavelengths. Higher emissivity increases the thermal radiation at any given temperature, whereas lower emissivity reduces the thermal radiation at the same temperature. A perfect thermal radiator is called a black-body, and has an emissivity of 1.0. Thermal radiators that are not perfect and that emit thermal radiation in the same spectral distribution as a black-body, but at reduced intensity, are called gray-bodies and have emissivities from zero to 1.0.
Emissivity can vary equally over the spectral range, called total emissivity; or it can vary as a function of the wavelength over the spectral range, called spectral emissivity. Both total emissivity and spectral emissivity can also vary as a function of the temperature of the substance. All of these emissivity variations can occur independently and simultaneously.
Scattering and absorption are two other important phenomena that affect intensity and distribution of the thermal radiation propagating through a medium, as well as the transmission distance (effective depth) through the medium. Scattering diffuses the thermal radiation, is highly dependent on both the species and physical size of the constituents of the medium, and increases significantly as the wavelength decreases. Absorption of thermal radiation is generally wavelength specific; the absorbed wavelength(s) being dependent upon the species that are exposed to the thermal radiation.
Optical pyrometry utilizes all of these radiating and propagating properties of matter to ascertain the temperature of a substance through a medium by measuring the intensity of the thermally radiated UV, VIS or IR energy of the substance. The radiating properties of matter are defined by Planck's equation for spectral emissivity and the Stefan-Boltzmann law for total radiated energy. The propagating properties through a medium are defined by the Boujuar-Lambert law for absorption and the Rayleigh and Mie equations for scattering.
Optical pyrometers can be either narrow bandpass or broad bandpass instruments. Typically, narrow bandpass optical pyrometers utilize Planck's equation to determine temperature, whereas broad bandpass optical pyrometers use the Stefan-Boltzmann law. Optical pyrometers are also classified according to the particular wavelength(s) utilized in determining the measured temperature, i.e., UV, VIS, or IR. Usually, the shorter wavelengths (UV, VIS and near IR) are used for higher temperature measurements, whereas longer wavelengths (middle and far IR) are used for lower temperature measurements.
Successful implementation of any optical pyrometer requires an extensive analysis of the application to determine the proper wavelength, bandpass and emissivity based on the characteristics of the measured substance over the desired measurement temperature range. Additionally, scattering and absorption in the propagating medium must also be considered to ensure that the measured thermal radiation is actually proportional to the measured temperature, and also to ensure the correct measurement depth into the propagating medium. This is especially important for boiler/furnace applications because the measured substance according to the present invention, flue-gas, is quite nebulous and its constituents can vary widely over normal operating conditions.
Without conducting the proper application specific analysis, most optical pyrometers cannot reliably and accurately measure boiler/furnace fireside gas temperatures. Typical VIS and broad bandpass IR type devices have a very short effective depth, and only measure the near-field temperature. Typical narrow bandpass IR type devices have an extremely long effective depth, and are greatly influenced by the opposite wall surface temperature.
U.S. Pat. Nos. 5,112,215 and 5,275,553, which are both incorporated here by reference, disclose optical pyrometers for measuring temperature based on single and double wavelength measurements of radiation from fly ash in a furnace.